Spitting any any other person is, as you say, an send off offense. If the spitting is directed at an opponent, a Direct Free Kick is awarded to the opposing team at the spot of the infraction. As a result, this would become a penalty kick if it happened in the opposing team's penalty area.
The offending player is given an excessive time out penalty. The penalty carries 5 yards for either offense or defense.
yes if it on the green
YES, THEY JUST REPLAY THE DOWN
A throw-in is not a penalty kick offense. It is a normal method of restarting play.
A team will draw a penalty if there aren't 11 players on the field at the time of the snap.
In the NFL, the penalty is 15 yards. In college, the penalty is 10 yards.
They can decline the penalty, yes... but there would be no reason to decline it, because in high school this is a dead ball foul and the play is immediately blown dead.
Most penalties result in replaying the down. Sometimes it is more beneficial to take the result of the play and not replay the down. Example, on 3rd and 10 there is a holding penalty on the offense during a 2 yard run. Accepting the penalty makes it 3rd and 20, declining makes it 4th and 8. On 4th down the offense should punt.
Yes. nearly any defensive penalty BEFORE change of possession (INT, Fumble recovery) can result in a free play for the offense.
In American football, when a defender catches a forward pass from the offense, it is called an interception.
In the UK yes they are.